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PartTimeProjects F5 Power Amplifier

The F5 is a stereo power amplifier from First Watt, a side project of Nelson Pass of Pass Labs. It is a 25 watt per channel Class A MOSFET power amplifier. Favorable reviews have been written about this amp, and it appears to be easy to build, having very few parts. There is a full article describing the operation and build of the amp.

I first started with an old 1980s Onkyo receiver for the case. Note how the faceplate is flat when all the knobs and switches are removed? This is ideal for mounting any flat metal or plastic faceplate onto the existing structure without creating any gaps. Today receivers have curved faceplates which create problems when you try to reuse the case/ chassis. The Onkyo receiver was completely taken apart and all components were removed. I cleaned it and all of its screws, and painted the cover and front bezel outside surfaces black. I think the brown "wood veneer" finish went out of style decades ago. New connectors were put on the rear panel. The old rear panel connectors that were removed were replaced with black plastic from a plastic sheet, rough on the inside but nice and shiny on the outside. This plastic is very thick and strong, but easy to dill compared to metal. I used the original speaker fuses from the Onkyo receiver (on the back panel) and designed a front panel from Front Panel Express using their software and the existing screw holes. It came out looking quite nice.

Original case with components removed
New back panel
Front Bezel edges spraypainted

I also built a voltage regulator to run a pair of fans. This uses an LM338 voltage regulator and the variable voltage regulator with protection diodes shown on page 6 of the LM338 data sheet. The voltage regulator input is connected to the +24 volts supply prior to the resistor in the CRC supply. That way any noise added by the regulator is reduced by the CRC supply before feeding the audio circuits. The LM338 takes the voltage from +24 volts down to +12 volts and lower. It only needs to apply about 200 mA to the fans, and it can supply 5A. I have it set for about 5 volts where you can barely hear the fans. The regulator does not even get warm, and I have mounted it to a heatsink but found that this was not necessary. Computer fans have an inset ledge on one side, you can actually mount them flush to the mounting screws if the nut and lockwasher are small enough to fit into the inset. I had very little clearance but it was no problem to mount these fans flush, turns out I had the right hardware sitting around. Lucky me.

Regulator Top
Regulator Bottom
Mounting the Fans Nice and Flush

I implemented the CRC power supply from the article and each amp channel using pcbs made by cviller. The heat sinks are about 5" tall, 8" wide and 1/4" thick, the fins extend about 2 inches. And these get really hot! I added a pair of fans, which are 70mm x 70mm and rated for about 25 CFM / 12 volts / 37 dBA. I am able to reduce fan speed and also noise using the variable regulator, and still keep the sinks at less than 60 degrees C.

Top View of F5 Power Amplifier
One Channel on Heatsink
Rear View

This amplifier was very easy to build. The power supply is simple. The Amplifier circuit is simple. The parts are straightfoward. You do need huge heat sinks, capacitors and and a big transformer, which makes this amplifier expensive (for 25 WPC) to build. And its a monster amp, large, heavy and gets very hot.

The front panel came in from Front Panel Express and fit perfectly. I mounted it up and the amp is finished.

Front Panel
Front Panel on Bezel
F5 Front View
F5 Back View

Feelings about the F5 and Low Power Amplifiers

So what do I think? My opinion is that the Pass Labs and First Watt DIY amplifiers are not usable by most people. I think people are used to amplifiers ranging from 50-100 WPC, and using speakers with efficiencies from 87-90 dB (1W, 1m). The Pass and FirstWatt DIY amps are 25-30 WPC or less (without special modifications). Pass Labs makes powerful commercial amplifiers, but in my view its DIY amps (without modification) need sensitive speakers or are only useful for specialized applications. Say background music or an office stereo (but you don't need a huge class A heater in your office). For a big and hot amp, the F5 lacks the authority that most people would expect. Reviews of these amps stress that the sound very powerful for "only" 25-30 WPC, but, after thinking about this for awhile, I am not sure I agree.

For me and my speakers (89 dB sensitivity), the F5 is useless. It cannot play even at moderate volume and there is no deep bass; bass is pathetic. I have dismantled the F5 to use the case for another project, the boards are sold to another DIYer. I have vowed to myself that I will no longer build any amplifier that is less than 50WPC. They just are not practical, even if they are fun to build, and the F5 is very very easy to construct. But to all of you potential builders out there- be aware that amplifiers less than 50 WPC may not be usable with typical speakers in a typical situation. Of course there are exceptions but don't say I didn't warn you. Also, if you are building for fun and to learn, you may not care about the practical use of such an amplifier, and the F5 is fun and easy to make.

UPDATE (2012): You will now see in the FirstWatt "Articles" Section an article about a higher power "F5 Turbo." I have no experience with this amplifier (since I am not diying anymore and no longer into audio like I used to be.) All I can say is "about time!" Click here.